Are you sure its broken - not just seized/stuck? or even come off the gear linkage at one end or the other.
IMHO - not necessarily poor technique - but possibly poor maintainence. You have a cable running inside a plastic outer - it gets salt, sand, grime inside it, it corrodes a bit and it gets jammed. If something has broken - probably someone forced it becuase it was already stuck/too-stiff. If you really want to point the finger at someone it should be at whoever didn't look at the cables when people started complaining about them being stiff.
Some other reasons for stiffness - cables too long for the boat, cables routed too sharply/tightly in the boat, adjuster screw on the contol leaver set too tight.